The present invention relates to a direct conversion receiver in which the radio frequency is directly converted to a baseband signal.
In a conventional superheterodyne receiver, high-frequency and intermediate-frequency filters both of high-selectivity type are required to remove image frequencies. To achieve small size and low assembly cost, direct frequency conversion technique has been employed in a homodyne receiver in which the radio frequency of the received signal is converted directly to a baseband frequency in a manner as shown and described in U.S. Pat. Nos. 4,193,034 and 4,254,503. With the direct conversion receiver, orthogonal, or quadrature local carrier frequencies are tuned to the transmitted carrier to provide zero intermediate frequency (homodyne) and baseband signals are detected by mixers as beats in quadrature between the transmitted carrier and the local carriers in quadrature. The outputs of the mixers are passed through low-pass filters to remove noise, and fed through limiting amplifier stages a discriminator. Because of the zero intermediate frequency, no need exists to remove image frequencies which are present in conventional superheterodyne receivers in which high-selectivity filters are used in the high-frequency and intermediate frequency amplifiers. Another advantage of the zero intermediate frequency is that the channel filter for attenuating signals from adjacent channels can be implemented with low-frequency active filters on integrated circuits.
However, one disadvantage is that the bandwidth in which acceptable level of sensitivity can be obtained is narrow. Since the amount of information contained in a single bit, E.sub.B, is given by E.sub.B =2.times.F.sub.D /B.sub.R, (where F.sub.D is maximum frequency deviation as a modulating digital signal varies between a mark and a space, and B.sub.R represents the transmission speed), and the beat frequency F.sub.B is represented by F.sub.B =(F.sub.C .+-.F.sub.D)-F.sub.L (where F.sub.C and F.sub.L are the transmitted carrier frequency and the local carrier frequency, respectively). If there is no difference between F.sub.C and F.sub.L, then the beat frequency F.sub.B is equal to the maximum frequency deviation F.sub.D and the marks and spaces have equal values for information quantity E.sub.B. However, if there is a frequency offset .DELTA.F between the transmitted and local carriers due to tuning errors, beat frequency F.sub.B is given by F.sub.B =.+-.F.sub.D -.DELTA.F. Therefore, beat frequency F.sub.B varies between F.sub.D -.DELTA.F for a "mark" and F.sub.D +.DELTA.F for a "space". This implies that information quantity E.sub.B iss les for "marks" than for "spaces", and hence, a low signal to noise ratio.